1. Field
The present invention relates to a phase change memory device, and more particularly, to a phase change memory device using a carbon nanotube to allow operation at low power and large scale of integration, and a method for fabricating the same.
2. Description of the Background Art
A phase change memory device is a type of memory that stores information using an electrical conductivity difference between a crystalline phase and an amorphous phase of a specific material.
Such a phase change memory device has received a great attention as a next generation non-volatile memory due to its unique characteristics such as large threshold voltage margin, rapid operation speed, excellent durability, and long data retention time. Furthermore, many researchers have recently reported about successful mass production of phase change memory devices that can be scaled equivalent to commercial flash memory devices.
The size of memory cells and operation of memory cells need to be maintained uniform in order for phase change memory devices to become a major next generation memory. The magnitude of operation current affects the size of the memory cells, and a contact area between a phase change material and a bottom electrode affects the magnitude of the necessary operation current. Therefore, the contact area between the phase change material and the bottom electrode is aimed to be reduced so as to have high density of the operation current with a small amount of operation current.
However, when etching is performed to form a bottom electrode in a phase change memory device, it is usually difficult to form a contact between the bottom electrode and a phase change material with a uniform diameter. Also, the size of the contact needs to be small to have high current density; however, in addition to the difficulty in obtaining the uniform contact size, downsizing the contact is another limitation in fabricating highly integrated phase change memory devices. As a result, improving reliability of phase change memory devices and the scale of integration may be limited.
Two approaches are suggested to achieve the large scale of integration in phase change memory devices based on the reduction in the size of the memory cells.
First, the size of the memory cells of the phase change memory devices can be reduced by reducing the contact area between the phase change material and the bottom electrode.
Second, resistance of the bottom electrode, which acts as a heating material, is increased to generate a large amount of heat under the same current density. As a result of this heat generation, the size of the memory cells of the phase change memory devices can be reduced. According to the known Joule definition, heat transferred to the phase change material is proportional to the resistance of a heat generating material and to the square of an amount of current flowing through the heat generating material.
On the basis of the above facts, a structure of a typical phase change memory device will be described hereinafter.
Small openings are formed in a bottom portion of a phase change material, and a bottom electrode, which is a heat generating material, fills the openings. As a result, the contact area between the bottom electrode and the phase change material is two-dimensional. In the typical phase change memory device, since operation current supplied from outside flows widely as much as the contact area, it is often difficult to obtain an amount of heat sufficient to cause a phase transition.
Hence, a method of forming a ring-type contact is introduced to overcome the above difficulty. In a phase change memory device using this ring-type contact, a heat generating material, i.e., the bottom electrode, fills only the surface of small openings, and an insulation material fills the rest of the small openings.
FIG. 1a is a perspective view of a typical phase change memory device structure. FIG. 1b is a sectional view of the typical phase change memory device structure cut in a 1b-1b′ direction illustrated in FIG. 1a. FIG. 1c is a top view of a bottom electrode of the typical phase change memory device.
Referring to FIG. 1a, in the typical ring-type phase change memory device, an external current source electrode 101 that supplies external current to a target and a phase change material layer 105 that shows the characteristics of the phase change memory device face to each other in side direction. A bottom electrode 102, which is a heat generating material, is formed in a ring shape between the external current source electrode 101 and the phase change material layer 105. An insulation material 103 fills the inside of the bottom electrode 102 in the form of a circle to prevent loss of heat outside. A dielectric material 104 encompasses the bottom electrode 102 and the insulation material 103.
In the typical ring-type phase change memory device, the phase change material layer 105 and the bottom electrode 102 contact with each other one-dimensionally in circumference. Thus, as compared with the typical phase change memory device showing the two-dimensional surface contact between the phase change material and the bottom electrode, the ring-type phase change memory device can have high density of operation current even with a small amount of the operation current. As mentioned above, since the insulation material 103 encompasses the bottom electrode 102, the loss of heat generated at the bottom electrode 102 can be blocked.
However, the bottom electrode 102 in the ring-type phase change memory device needs to fill inside of the small openings, and thus, a material for the bottom electrode 102 is selected with limitation. Also, despite of the ring formation method, the scale of integration in the ring-type phase change memory device is usually 50% of that of a currently fabricated flash memory. Hence, even with this ring-type phase change memory device structure, achieving the same or greater scale of integration is limited.
Accordingly, another phase change memory device structure that requires a small amount of operation current needs to be developed.